linux-kernel - Re: [PATCH v3 3/3] Force all cpus to exit VMX root operation on crash/panic reliably

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Message-ID: <CALCETrWnef+Q=Pyrf1h5tcPSrp7tW6eSVozjfONC+OsqbGcj-Q@mail.gmail.com>
Date:   Sun, 5 Jul 2020 13:53:39 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "David P. Reed" <dpreed@...pplum.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Allison Randal <allison@...utok.net>,
        Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Martin Molnar <martin.molnar.programming@...il.com>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/3] Force all cpus to exit VMX root operation on
 crash/panic reliably

On Sun, Jul 5, 2020 at 1:00 PM David P. Reed <dpreed@...pplum.com> wrote:
>
>
>
> On Sunday, July 5, 2020 2:26pm, "Andy Lutomirski" <luto@...nel.org> said:
>
> > On Sat, Jul 4, 2020 at 1:38 PM David P. Reed <dpreed@...pplum.com> wrote:
> >>
> >> Fix the logic during crash/panic reboot on Intel processors that
> >> can support VMX operation to ensure that all processors are not
> >> in VMX root operation. Prior code made optimistic assumptions
> >> about other cpus that would leave other cpus in VMX root operation
> >> depending on timing of crash/panic reboot.
> >> Builds on cpu_ermergency_vmxoff() and __cpu_emergency_vmxoff() created
> >> in a prior patch.
> >>
> >> Suggested-by: Sean Christopherson <sean.j.christopherson@...el.com>
> >> Signed-off-by: David P. Reed <dpreed@...pplum.com>
> >> ---
> >>  arch/x86/kernel/reboot.c | 20 +++++++-------------
> >>  1 file changed, 7 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
> >> index 0ec7ced727fe..c8e96ba78efc 100644
> >> --- a/arch/x86/kernel/reboot.c
> >> +++ b/arch/x86/kernel/reboot.c
> >> @@ -543,24 +543,18 @@ static void emergency_vmx_disable_all(void)
> >>          * signals when VMX is enabled.
> >>          *
> >>          * We can't take any locks and we may be on an inconsistent
> >> -        * state, so we use NMIs as IPIs to tell the other CPUs to disable
> >> -        * VMX and halt.
> >> +        * state, so we use NMIs as IPIs to tell the other CPUs to exit
> >> +        * VMX root operation and halt.
> >>          *
> >>          * For safety, we will avoid running the nmi_shootdown_cpus()
> >>          * stuff unnecessarily, but we don't have a way to check
> >> -        * if other CPUs have VMX enabled. So we will call it only if the
> >> -        * CPU we are running on has VMX enabled.
> >> -        *
> >> -        * We will miss cases where VMX is not enabled on all CPUs. This
> >> -        * shouldn't do much harm because KVM always enable VMX on all
> >> -        * CPUs anyway. But we can miss it on the small window where KVM
> >> -        * is still enabling VMX.
> >> +        * if other CPUs might be in VMX root operation.
> >>          */
> >> -       if (cpu_has_vmx() && cpu_vmx_enabled()) {
> >> -               /* Disable VMX on this CPU. */
> >> -               cpu_vmxoff();
> >> +       if (cpu_has_vmx()) {
> >> +               /* Safely force out of VMX root operation on this CPU. */
> >> +               __cpu_emergency_vmxoff();
> >>
> >> -               /* Halt and disable VMX on the other CPUs */
> >> +               /* Halt and exit VMX root operation on the other CPUs */
> >>                 nmi_shootdown_cpus(vmxoff_nmi);
> >>
> >>         }
> >
> > Seems reasonable to me.
> >
> > As a minor caveat, doing cr4_clear_bits() in NMI context is not really
> > okay, but we're about to reboot, so nothing too awful should happen.
> > And this has very little to do with your patch.
>
> I had wondered why the bit is cleared, too. (I assumed it was OK or desirable, because it was being cleared in NMI context before). Happy to submit a separate patch to eliminate that issue as well, since the point of emergency vmxoff is only to get out of VMX root mode - CR4.VMXE's state is irrelevant. Of course, clearing it prevents any future emergency vmxoff attempts. (there seemed to be some confusion about "enabling" VMX vs. "in VMX operation" in the comments)  Should I?

I have a vague recollection of some firmwares that got upset if
rebooted with CR4.VMXE set.  Sean?

The real issue here is that the percpu CR4 machinery uses IRQ-offness
as a lock, and NMI breaks this.

--Andy